This invention relates generally to techniques for selectively detecting photons and, more specifically, to techniques for detecting a weak omnidirectional and monochromatic photon signal by atomic resonance filtering. Atomic resonance filters are important components of laser communications receivers, but they have a relatively low throughput efficiency, or photon signal to photoelectron conversion efficiency. This low throughput efficiency can result from unfavorable quantum mechanics in the filter process, losses due to coupling of frequency-shifted photons out of the filter, and losses due to low quantum efficiency of a photocathode used in the detector.
One approach to narrow bandwidth filtering with wide acceptance angles is to use atomic absorption/fluorescence filters, such as are described in a paper by Marling et al., J. Appl. Phys. 50, 610 (1979). Atomic absorption/fluorescence narrow bandwidth infrared up-converters are described in a paper by Gelbachs et al., IEEE Trans. on Electron Devices, VOL ED-27, 99 (1980). The fluorescence principle is one of photon-to-photon conversion. Photons of a selected wavelength are converted by absorption and fluorescence to photons of another wavelength.
U.S. Pat. No. 4,600,840 to Chutjian discloses a filter employing a principle of electric field ionization. Signal photons are absorbed by atoms or molecules of a vapor in a detector cell, by which process the atoms or molecules are raised to a higher-energy metastable state. Then, by application of a powerful electric field, electrons are detached from the metastable particles, to produce ions and measurable electrons. The principal disadvantage of this approach is that a very large electric field, measuring several thousand volts per centimeter, is needed for ionization.
U.S. Pat. No. 4,879,468 to Brock, entitled "Photoionization Optical Filter and Detector," discloses a narrowband filter using a photoionization principle. Signal photons admitted into a detector cell selectively excite atoms or molecules of a vapor to a selected metastable state; then an illuminating laser photoionizes a large proportion of the metastable atoms or molecules, to produce measurable ions and electrons. No large electric field is needed for this process, but a laser is needed for ionization. The present invention provides an improvement over the photoionization filter, and needs neither a high electric field nor an ionizing laser.